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Replication Policies

iOS replication policies

Replication policies on iOS run in a background task to allow them to run independently of the rest of your application and to enable them to continue even when the app is suspended or terminated.

It is also possible to configure periodic replication of data whilst the app is in the background. It is important to realise that iOS imposes restrictions on background data transfer and these apply to replication policies. In particular, iOS limits the time during which data transfers can occur while the app is in the background to 30 seconds. Also, although a minimum background fetch interval can be specified, this is the absolute minimum period between performing background data transfers and the actual interval may be significantly different to the minimum interval specified.

Configuring background execution

To configure background execution, it is necessary to:

  • enable the Background fetch option for the Background modes section of the Capabilites tab in your Xcode project (you can also enable this support by including the UIBackgroundModes key with the fetch value in your app's Info.plist file).
  • implement an app delegate method application:performFetchWithCompletionHandler: to initiate the replications. Once replications are complete, this method must execute the provided completion handler block, passing a result that indicates whether content was available.
  • set the minimum background fetch interval by calling setMinimumBackgroundFetchInterval: on the application's UIApplication object. The value given to setMinimumBackgroundFetchInterval: is advisory only and the actual time between background replications may vary considerably from the value set.

You should note that enabling background fetch does not guarantee that iOS will give your app any time to perform background fetches as iOS will attempt to balance your app's need to fetch content with the needs of the system and of other apps.


Let's assume we wish to configure a replication policy as follows:

  • We only ever want replications to occur when the device is connected to a WiFi network.
  • We want to do sync replications (pull and push).
  • When the app is not displaying data to the user we want replications to occur once every 24 hours to keep the data on the device fairly fresh.
  • When the app is displaying data to the user we want replications to occur every 5 minutes so the data displayed to the user is only ever a few minutes out of date if we're on Wifi.
  • When the app is displaying data to the user we want to refresh the UI to display the new data when the pull replication has completed.
  • After the device has rebooted, we want replications to continue in the same way as prior to the reboot.

Configuring app capabilities

On the Capabilities tab of your Xcode project turn on Background Modes and select Background fetch.

Configure your replications

Configure your replications following the guidance in the Replication guide.

Note however, that in the following example we'll use the startWithTaskGroup:error: method of CDTReplicator instead of startWithError: so that we can batch the replication tasks and wait for the completion of all of them.

For example:

/** Start both a push and pull replication.
 *  @param taskGroup A dispatch_group_t to allow us to wait for replications to complete.
- (void)startReplications:(dispatch_group_t)taskGroup
    // Create the replicator factory
    CDTReplicatorFactory *replicatorFactory =
       [[CDTReplicatorFactory alloc] initWithDatastoreManager:manager];

    // username/password can be Cloudant API keys
    NSString *s = @"";
    NSURL *remoteDatabaseURL = [NSURL URLWithString:s];
    CDTDatastore *datastore = [manager datastoreNamed:@"my_datastore"];

    // Create a replicator that replicates changes from the local
    // datastore to the remote database.
    CDTPushReplication *pushReplication = [CDTPushReplication replicationWithSource:datastore

    // Create a replicator that replicates changes from a remote
    // database to the local one.
    CDTPullReplication *pullReplication = [CDTPullReplication replicationWithSource:remoteDatabaseURL

    NSError *pushError;
    CDTReplicator *pushReplicator = [replicatorFactory oneWay:pushReplication

    NSError *pullError;
    CDTReplicator *pullReplicator = [replicatorFactory oneWay:pullReplication

    // Start the push replication
    if (![pushReplicator startWithTaskGroup:taskGroup error:&error]){
        //handle error

    // Start the pull replication
    if (![pullReplicator startWithTaskGroup:taskGroup error:&error]){
        //handle error

Create a method to start replications and wait for their completion (on a background thread).

-(void)syncInBackgroundWithCompletionHandler:(void (^)())completionHandler
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        dispatch_group_t backgroundTasks = dispatch_group_create();

        UIBackgroundTaskIdentifier taskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:NULL];

        [self startReplications:backgroundTasks];

        dispatch_group_wait(backgroundTasks, dispatch_time(DISPATCH_TIME_NOW, 30 * NSEC_PER_SEC));

        if (completionHandler) {

        [[UIApplication sharedApplication] endBackgroundTask:taskId];

Setup an NSURLSessionConfigurationDelegate to customise the NSURLSession as you require:

- (void)customiseNSURLSessionConfiguration:(nonnull NSURLSessionConfiguration *)config {
    config.allowsCellularAccess = NO; // Wifi only.
    config.sessionSendsLaunchEvents = YES;

Setting up the App Delegate

In your UIApplicationDelegate add the following to your application:didFinishLaunchingWithOptions: method:

int backgroundInterval24hrs = 24*60*60;
[[UIApplication sharedApplication] setMinimumBackgroundFetchInterval:backgroundInterval24hrs];

Implement the application:performFetchWithCompletionHandler: method - e.g.:

- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
    [self.myReplicator syncInBackgroundWithCompletionHandler:^{
        // Do whatever you want when replication is complete (e.g. refresh the UI)

        // Call the completion handler once the sync has finished.

Configure replication when a particular screen is visible

In your screen's view controller, add methods to start and stop the timer performing replications:

@property (nonatomic, strong) NSTimer *timer;

- (void)startTimer:(id)sender {
    _timer = [NSTimer scheduledTimerWithTimeInterval:30

- (void)stopTimer:(id)sender {
    [_timer invalidate];
    _timer = nil;

- (void)periodicReplication:(NSTimer *)timer
    [self.todoReplicator syncInBackgroundWithCompletionHandler:^{
        // Do whatever you want when replication is complete (e.g. refresh the UI)

Add the following to the View Controller's viewWillAppear:animated method:

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(stopTimer:) name:UIApplicationWillResignActiveNotification object:nil];

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(startTimer:) name:UIApplicationWillEnterForegroundNotification object:nil];